FIG. 1 is a schematic structural diagram of a passive optical network (PON) system 100. As shown in FIG. 1, the system 100 includes the following three parts: an optical line terminal (OLT) 102, an optical distribution network (ODN) 104, and an optical network unit (ONU) 106. In the PON system 100, transmission from the OLT 102 to the ONU/ONT 106 is referred to as downlink, and transmission from the ONU/ONT 106 to the OLT 102 is referred to as uplink. Downlink data is broadcast by the OLT 102 to each ONU 106 because of a characteristic of light. A transmit timeslot is allocated by the OLT 102 for sending uplink data of each ONU 106. Time division multiplexing transmission is used in an uplink direction. The ODN 104 is an optical distribution network, which transmits downlink data of the OLT 102 to each ONU 106 and collects and transmits uplink data of multiple ONUs 106 to the OLT 102. The ONU 106 provides a user-side interface to the PON system 100, and is connected to the ODN 104 in uplink. The ODN 104 is generally divided into three parts: a passive optical splitter (Splitter) 108, a feeder fiber 110, and a distribution fiber 112. For a general PON system 100, different wavelengths are used in downlink and uplink. A direction from an OLT 102 to an ONU 106 is referred to as a downlink direction, and a center wavelength of 1490 nm is used in a G/EPON (Gigabit passive optical network/Ethernet passive optical network). A direction from the ONU 106 to the OLT 102 is referred to as an uplink direction, and a center wavelength of 1310 nm is used in the G/EPON.
A PON 100 is of a tree structure, and there are multiple ONUs 106 connected to an OLT 102 of one central office. Therefore, how to maintain network stability and how to determine fault liability become current focuses of attention.
Currently, a common means in the industry is performing fault detection and locating in an optical network by using an optical time domain reflectometer (OTDR). The basic principle of the optical time domain reflectometer is that light of a wavelength is incident into a fiber network by means of backward reflection generated when an optical wave is propagated in the fiber network, and then, an optical network status is reflected by measuring energy of corresponding reflected light, which, for example, is described by using the prior art 1 in FIG. 2 as an example. Downlink light is of 1490 nm, uplink light is of 1310 nm, and the light of 1310 nm can penetrate through a TFF filter 202, enter a channel b, and be detected by a photo detector (PD). The downlink light of 1490 nm carries an OTDR detection signal, enters a PON from a channel a after being reflected by a thin film filter (TFF), after a reflected optical signal of the PON returns to the channel a, enters a channel d after being reflected by the TFF filter, and is detected by the PD of 1490 nm. A speed of light in a fiber can be estimated, and a curve of reflected light intensity that changes over time corresponds to a curve of the reflected light intensity that changes with distance. Therefore, a particular fault that occurs at a particular distance can be determined according to a change of the reflected light intensity. For example, that a large amount of reflected light energy is detected means that a problem of fiber cut may occur at a particular distance. If energy decrement is detected, it means that a problem of fiber bending may occur, and further fault rectification is performed.
For a TWDM-PON (Time Wavelength Division Multiplexing Passive Optical Network) as a next generation PON technology, there is no solution for how to determine a fiber fault by using an OTDR.